In the past, metal detectors have suffered from the inability to distinguish between ferrous and non-ferrous objects in the ground, especially when the ground contains a ferrous, mineralized soil background such as black sand or a ferric oxide bearing soil. This mineralized soil background electromagnetically resembles a ferrous object, tends to mask ferrous objects from detection and "confuses" the metal detector, resulting in false outputs. Metal detectors in the past have generated an output when no objects were present because of the presence of a ferrous, mineralized soil background. This is very undesirable since the paramount goal for the use of the metal detector is to locate metallic objects in the ground, and there are many areas where ferrous, mineralized soil is present which frustrates the use of known metal detectors.
In addition, sampling metal detectors which employ low sampling rate suffer from the inability to nullify the effect of relative motion between the metal detector and the ground containing the objects to be detected. This effect causes a stronger signal to be generated by the metal detector as it is moved closer to the ground and a weaker signal to be generated as the detector is moved further away from the ground. This results in confusion and frustrates the operation of the detector because stronger or weaker signals also may indicate the size, mass or depth of an object when there is no relative motion between the detector and the ground.
It is therefore the primary object of the present invention to provide an improved metal detector capable of detecting and distinguishing between ferrous and non-ferrous objects in an area which may contain a ferrous mineralized soil background.
Another object of the invention is to provide such a metal detector which employs the hysteresis effect and sampling to distinguish between ferrous and non-ferrous objects and to eliminate the effect of mineralized soil.
It is still another object of the present invention to provide an improved metal detector which nullifies the effect of relative motion between the metal detector and the ground.
It is a further object of the present invention to provide a metal detector with an adjustable electronic sampling circuit to eliminate the effects of a ferrous mineralized soil background on the metal detector.
It is a still further object of the present invention to provide an improved metal detector including a balanced synchronous demodulator circuit for nullifying the effect of relative motion between the metal detector and the ground.